1. Field of the Invention
The present invention relates to a magneto-resistive element, a tunneling magneto-resistive element, a magnetic head device using the element, a magnetic recording/reproducing apparatus using the magnetic head device, and a method for manufacturing the element.
2. Description of the Related Art
Tunneling magneto-resistive (TMR) elements have been developed as a magneto-resistive (MR) element with high magneto resistance ratio. In tunneling magneto-resistive elements, an ultra-thin barrier layer, which is mainly constituted of metal oxide, is disposed between a free layer and a pinned layer. Above and below a tunneling magneto-resistive film, which is a lamination of an antiferromagnetic layer, the pinned layer, the barrier layer and the free layer, there are disposed a pair of magnetic shielding layers to prevent degradation of reading characteristics due to external magnetic field.
For the barrier layer, there have been widely used amorphous oxides such as aluminum oxide, titanium oxide, tantalum oxide, zirconium oxide and hafnium oxide. In recent years, however, it has been found that MR ratio can be significantly enhanced by constituting the barrier layer of crystalline magnesium oxide (MgO), as described in “Single-Crystal TMR (Tunneling Magneto-Resistive) Element with World's Highest Performance,” AIST Today, vol. 4, no. 5, pp. 4-6, National Institute of Advanced Industrial Science and Technology (May 2004).
In addition, attempts have been made to enhance MR ratio of current perpendicular to plane type giant magneto-resistive (CPP-GMR) element by using a half-metal material with high polarizability (e.g., Heusler alloy).
In the conventional case where the barrier layer is constituted of amorphous oxides, annealing at a temperature of 250 to 300° C. is sufficient to stabilize the exchange coupling between the antiferromagnetic layer and the pinned layer or to stabilize the barrier layer. In the case where the barrier layer is constituted of magnesium oxide for enhancement of MR ratio, on the other hand, it is preferred that annealing is performed at a higher temperature to obtain sufficient MR ratio by enhancing crystallinity. Also in the CPP-GMR element, annealing at a higher temperature is desirable to obtain sufficient MR ratio by ordering of Heusler alloy. However, high-temperature annealing may grow coarse grains within the magnetic shielding layer, affecting the adjacent tunneling magneto-resistive film to have irregularities, thereby causing a problem of variance in characteristics.